1. Field of the Invention
The present invention relates to display devices for displaying color images, and more specifically to display devices that generate a gradation voltage group made of voltages representing the gradations of an image, and that displays color images using a voltage that is selected from this gradation voltage group in accordance with an input signal, as well as driving circuits of such display devices.
2. Description of the Related Art
Liquid crystal display devices, for example, are provided with a gradation voltage generation circuit generating voltages representing the gradations in order to perform a gradation display. One of the plurality of voltages generated by this gradation voltage generation circuit is selected in accordance with an input signal, and an image of intermediate gradations is displayed by applying the selected voltage as the driving signal to the liquid crystal panel.
As described for example in JP 2002-82645A (the content of the corresponding U.S. 2001/0052897A1 is hereby incorporated by reference), a gradation voltage generation circuit for such gradation display is typically incorporated in a video signal line driving circuit (referred to as “column electrode driving circuit”) for driving the liquid crystal panel, and is realized as a voltage divider circuit that is a resistor series made of a plurality of resistors connected in series. The gradation voltages depend on the voltage division ratios of this voltage divider circuit, and it is important to set the voltage division ratios with consideration to the display quality.
A color filter that is used for the color image display with a liquid crystal display device is typically made of filters of the three colors R (red), G (green) and B (blue) constituting three primary colors, but as shown in FIG. 9, the gradation level—brightness curve is slightly different for each of these three colors. This means, that the gradation reproducibility of the pixel formation portions constituting the liquid crystal panel is different for each of the three colors. In FIG. 9, the horizontal axis marks the gradation levels of the colors R, G and B represented by the input signal, and the vertical axis marks the brightness of the colors R, G and B on the liquid crystal panel. It should be noted that the brightness on the vertical axis is normalized by its maximum value.
Thus, the gradation level—brightness curves are slightly different for each of the colors R, G and B, but in conventional liquid crystal display devices, the gradation voltage generation circuit is provided with only one resistor series, or two resistor series for positive polarity and negative polarity (in the following, it is assumed for convenience's sake that there is only one resistor series, even if a resistor series for positive polarity and a resistor series for negative polarity are provided). Therefore, it was not possible to set the gradation voltages (or voltage division ratios) individually in accordance with the gradation level—brightness curve for each of the colors R, G and B. As a result, it was not possible to maintain a superior color balance across the entire brightness region, and a high degree of color reproducibility could not be attained. Moreover, in ordinary liquid crystal display devices, if three resistor series corresponding to the gradation level—brightness curves of the colors R, G and B are provided, then three times the number of voltage bus lines for transmitting the gradation voltages are necessary (gradation number×3), and the chip area of the IC (integrated circuit) for realizing the video signal line driving circuit increases considerably.